The present invention relates to a method of inspecting an operation of sealed closure by welding the end of a filling channel traversing the upper plug of a nuclear fuel rod.
Nuclear reactors, such as nuclear reactors cooled by pressurized water, comprise a core consisting of fuel assemblies in which energy in the form of heat is produced during operation of the reactor.
Each of the fuel assemblies generally consists of a bundle of mutually parallel fuel rods held in a framework of the fuel assembly. Each of the fuel rods comprises a tubular padding made of a material which weakly absorbs neutrons such as a zirconium alloy, in which nuclear fuel pellets are stacked, for example sintered pellets of UO2 uranium. The tubular cladding is closed at its ends by plugs, each of which comprises a cylindrical part which is engaged coaxially in an end part of the cladding. The plug and the cladding are then fastened one to the other by welding around a circular line located in a plane which is substantially perpendicular to the axis of the cladding and of the plug.
One of the two plugs for closing the cladding of a fuel rod, which is called the upper plug, because it closes the upper end of the rod inside the fuel assembly in the service position of the fuel assembly in the core of the nuclear reactor, is traversed axially by a channel, wherein the cladding of the rod is filled, around the nuclear fuel pellets, by an inert pressurized gas such as helium which protects the fuel pellets against oxidation and promotes heat exchange between the pellets and the cladding of the rod when the rod is in service in the core of the nuclear reactor.
The manufacture of the fuel rods requires numerous successive operations in order to fill the cladding with the fuel pellets, to place and weld the plugs and to introduce an inert pressurized gas such as helium into the cladding sealed shut by the plugs, and to close the filling channel of the plug, after filling. Numerous inspections must be carried out at all steps of the fuel rod manufacture, so as to attain fuel rods which are completely free of defects.
In particular, sealing the closure by welding the filling channel of the upper plugs must be subject to rigorous inspection.
The filling with pressurized helium of the cladding of the rods containing the fuel pellets and sealed shut by the plugs is carried out in a filling apparatus in which the upper end part of the fuel rod, comprising the upper plug traversed axially by the filling channel, is inserted. The end of the upper plug, on which the filling channel emerges in the form a circular inlet opening extended axially by a chamfered part of the filling channel, is placed so as to be able to engage with a valve for closing and opening the chamfered end part of the filling channel. In the open position of the valve, the air contained inside the fuel cladding is evacuated, then the rod is filled by pressurized helium entering inside the cladding via the filling channel of the upper plug. Finally, in the filling apparatus, the end of the filling channel is sealed shut by a weld obtained by melting a central part of the plug adjacent to the end part of the filling channel. Generally, the material of the plug is melted to carry out the weld under the effect of a laser beam directed axially on the chamfered inlet part of the filling channel. The weld obtained by a pulsed laser is generally formed from three successive spot welds, in order to increase the safety and the production quality of the closure. The chamfered inlet part of the filling channel is generally known by the term xe2x80x9cseal weldxe2x80x9d.
The weld is carried out under satisfactory condition only if the axis of the laser beam for melting the material of the plug, along the periphery of the chamfered inlet part of the filling channel, is centered accurately with respect to the circular inlet opening of the seal weld and if this circular opening has a diameter the size of which is predetermined and defined very accurately.
Furthermore, after welding, the quality of the spot welds can be inspected, in order to determine whether the fuel rod complies perfectly.
The inlet opening of the filling channel or pressurization hole is positioned manually and with simple visual monitoring, before the welding operation. The mechanical adjustment for manually controlling the positioning of the plug and of the pressurization hole cannot enable the perfect positioning of the pressurization hole to be guaranteed in all cases during welding. There are possibilities of maladjustment or blockage of the rod, which result in poor positioning. In addition, the visual monitoring of the positioning of the filling channel does not make it possible to obtain a highly efficient adjustment either.
The quality of the spot weld is inspected by X-ray scanning. This inspection makes it possible to detect porosities in the weld but does not allow the position and size of the weld to be verified satisfactorily.
Furthermore, the inspection must be carried out on a separate station of the filling apparatus, which makes the rod manufacture operations more complex and longer.
Where the sealing weld is carried out by laser welding, the quality of the laser weld depends on the geometry of the chamfer of the inlet part of the filling channel; in particular, it is necessary to verify that the chamfer has an inlet opening whose diameter, which constitutes the maximum diameter of the chamfer, complies with extremely strict size requirements.
The aim of the invention is therefore to propose a method of inspecting an operation of sealed closure by welding an end opening of a filling channel axially traversing an upper plug for closing the cladding of a fuel rod for a nuclear reactor, the cladding of the rod containing a plurality of pellets of nuclear fuel stacked in the axial direction of the cladding and two closure plugs, one of the plugs or the upper plug being traversed axially by the channel for filling the cladding of the rod with an inert gas and the sealed closure by welding of the filling channel of the upper plug being carried out after filling the cladding with inert gas, in a filling apparatus, by melting central part of the end of the upper plug adjacent to the opening of the filling channel, this method allowing for inspection of the conditions for implementing and carrying out the sealed closure of the upper plug by welding, efficiently and without extending the time needed for the manufacture of the fuel rod.
With this aim, prior to the sealed closure of the filling channel, the fuel rod being in the position for filling and for sealed welding of the upper plug in the filling apparatus, images are acquired of the end of the plug on which the substantially circular inlet opening of the filling channel emerges, and in order to obtain a digitized image, the position of the center of the circular inlet opening of the filling channel is determined with respect to a reference position and the diameter of the inlet opening of the filling channel by analyzing the digitized image. It is thereby deduced whether it is possible to weld the filling channel. Where the sealed closure of the filling channel is carried out by welding, images are acquired of the end of the upper plug after welding and the presence and the position of a weld for sealed closure of the filling channel are determined.